Commercial membranes may be produced from a variety of polymeric materials. Examples of useful polymeric materials include poly sulfone (PS), polyether sulfone (PES), polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene(PP), polytetrafluoroethene (PTFE), polyamides (PA), polyimide (PI), cellulose acetates (CA), and cross-linked polyvinyl alcohols (XPVOH). Additional material examples can be found in Nanofiltration Principles and Applications (eds. Schafer; Fane; Waite, Elsevier, Oxford, UK, 2005.)
Many of the engineering thermoplastics, such as PS, PES, and PVDF, exhibit good mechanical, thermal, and chemical resistance properties but suffer from poor hydrophilic, wetting properties. The poor hydrophilicity results in the need for additional agents to improve the hydrophilicity of the substrate, thereby enabling the enhanced permeation of materials into such thermoplastics.
Several strategies can be employed to improve the surface wetting properties of engineering thermoplastics. First, the thermoplastic material can be treated by plasma polymerization of the surface with hydrophilic monomers such as N-vinyl-2-pyrrolidone (VP), vinyl pyridine, thiopene, and the like (Nanofiltration Principles and Applications). This method is not ideal due to the exposure of plant production personal to potentially hazardous materials and potential residual fugitive materials after processing. Second, DE-A 19817364 describes a process utilizing two different weight fractions of water soluble polymer. The low molecular weight material washes away, leaving behind the high molecular weight fraction as the hydrophilic additive. This method is not ideal because it does not account for the fact that the high molecular weight, water soluble material is still water soluble. Thus, over time, the hydrophilic agent can still solubilize, thereby reducing the overall permeability of the thermoplastic. Third, WO 94/17906 discloses the utilization of an interpenetrating network (IPN) strategy to entrap polyvinyl pyrrolidone (PVP), a water soluble polymer, into the membrane. However, the physical, solubility of PVP remains unchanged, if only delayed, dampening the long term usefulness of the permeable thermoplastic. This work illustrates the desire of the art to develop materials suitable for enhancing the long term, hydrophilicity of membrane thermoplastic materials.